The present invention relates to divergent ultrasound arrays of transducers adapted to treat medical conditions responsive to ultrasound such as physiotherapeutic conditions and the treatment of neoplastic tissues by ultrasound hyperthermia.
In physiotherapy it has been known to use an ultrasonic transducer in contact with the skin to treat various pain inducing conditions in the joints for example. The use of these ultrasound transducers of the convergent variety is limited because the transducer head must be in intimate contact with the skin, and in that the power output cannot be raised too far because local heating of the skin exceeds patient tolerence. Accordingly, only fairly low ultrasound intensities can be applied to affected parts; ultrasound treatments in depth are not possible.
Focused convergent arrays are also impractical because of the difficulties in providing an acoustic focus at an appropriate point having regard to the presence of bone etc., particularly with the added requirement that with most therapeutic conditions no invasive temperature sensors can be utilized.
Local hyperthermia, in which temperatures normally in the range between 42.degree. and 46.degree. C. are maintained for periods from a few minutes up to several hours in neoplastic tissue, has been found to be beneficial for the treatment of cancer in cases where the whole tumour can be properly heated and the surrounding normal tissues can be spared. Many methods of producing local hyperthermia in cancer patients have been suggested and tested. Recently treatment of superficial human neoplasms by local hyperthermia induced by ultrasound has been shown to be useful.
For ultrasonically-induced local hyperthermia, the elevation of temperature inside the target tissue is affected by the ultrasonic absorption, the blood perfusion rate, the thermal conductivity of the tissue, and the choice of transducer parameters. It has been shown that the temperature distribution produced by a single convergent focused ultrasonic beam is very sharp even in the steady-state situations where the thermal conduction and blood perfusion smooth it. Moreover, we have found that the treated volumes are usually not large enough for most of the clinical cases.
In commonly assigned U.S. patent application No. now U.S. Pat. No. 4,646,756, Watmough et al. have described an array of sonic transducers radially disposed about an axis, angled to direct sonic energy toward said axis at a point of acoustic focus remote therefrom, and temperature sensing means for association with the point of acoustic focus whereby one or more transducers are adapted for relative angular movement thereby to allow adjustment of the shape and/or position of the acoustic focus in response to an output from the temperature sensing means.
A system utilizing circulating warm water in conjunction with multiple convengent ultrasonic beams to achieve local hyperthermia has also been described in the above U.S. patent application. The disclosures of U.S. Pat. No. 4,646,756 are hereby incorporated by reference herein.
When a single steered, focussed ultrasonic beam is used, only a small portion of the tumour tissue is heated at one time, so the elevation of the temperature inside the tumour is completely dependent upon the averaging effect induced by thermal conduction and blood perfusion in tissue. The possibility of under-heating or over-heating a certain portion of tissue exists as the blood perfusion rate will change during treatment because of vasodilation. Furthermore, lack of homogeneity in blood flow and inhomogeneity in ultrasonic absorption in the tumour will greatly affect the temperature distribution. Moreover, a single steered and focussed ultrasonic beam takes a longer time to heat up a whole tumour to a therapeutic temperature level, and also involves much higher instantaneous intensities.
The array in accord with U.S. Pat. application Ser. No. 544,820, although effective, requires in practice the provision of computer controlled stepping motors to alter the focal points of the movable transducer array in response to outputs from said temperature sensing means, and this adds to the complexity and cost of the device.